A device operating in a radio transmission system typically has a radio frequency circuit (e.g. an RF chip) and a baseband circuit (e.g. a baseband chip), with a link allowing data to be sent between the radio frequency circuit and the baseband chip. The radio frequency circuit typically includes at least one antenna for transmitting and receiving radio signals over the radio transmission system. The radio frequency circuit operates at radio frequencies (of the order of 109 Hz) to transmit and receive the radio signals. The baseband circuit operates at the operating frequency of the device (typically of the order of 108 Hz), in accordance with a clock signal (having a clock frequency FC) provided in the device.
When a radio signal is received at the device it is typically demodulated in the radio frequency circuit to extract the data carried in the radio signal. The demodulated signal is then sent to the baseband circuit for use in the device. In some systems where the baseband circuit operates in the digital domain, the demodulated signal (which is an analogue signal) is passed through an analogue to digital converter in the radio frequency circuit before being sent to the baseband circuit over the link. Therefore digital signals are sent over the link. The demodulated data signal comprises at least one data stream signal (having a bit rate of 2FC) which can be sent over the link. In addition to sending the at least one data stream signal, a clock signal is sent over the link providing timing information allowing the baseband circuit to interpret the data stream signal(s).
Similarly, when signals are to be transmitted from the device via the antenna, the signals are sent from the baseband circuit to the radio frequency circuit over the link. The signal comprises a data signal (comprising at least one data stream signal) and a dock signal. On the radio frequency circuit the data signal is passed through a digital to analogue converter to convert the data signal into an analogue signal. The analogue signal is modulated in the radio frequency circuit and then transmitted over the radio transmission system from the device using the antenna.
One such system is described in the US patent application by Mohindra et al entitled “Serial Digital Interface for Wireless Network Radios and Baseband Integrated Circuits” published under publication number US 2005/0119025 A1 which discloses a system including a radio receiver integrated circuit, a radio transmitter integrated circuit and a baseband digital signal processing integrated circuit with a digital interface between the radio integrated circuits and the baseband digital signal processing integrated circuit.
The signals being sent on the link between the radio frequency circuit and the baseband circuit may generate spurious emissions, which may interfere with the transmission and reception of radio signals to and from the antenna. These spurious emissions add to the noise to which the radio signals are subjected at the antenna.
If strong noise is generated, within a radio frequency band at which the device is transmitting or receiving radio signals over the radio transmission system, by sending the clock signal or the data signal over the link, then there is a detrimental effect on the performance of the device.
It is an aim of the present invention to reduce detrimental effects of noise generated by the signals sent on the link between the radio frequency circuit and the baseband circuit.